Gas masks are used, for example, for fighting unintended fires that may cause damage and when handling hazardous substances and materials. Gas masks with the “normal pressure” and “overpressure” modes are known.
The “normal pressure” mode is present, for example, when a breathing filter is connected to the gas mask. The pressure level in the interior of the gas mask corresponds now to the atmospheric ambient pressure. The “overpressure” mode is present, for example, when a demand oxygen system of a compressed air breathing apparatus is connected to the gas mask. The pressure level in the interior of the gas mask is now higher than that of the atmospheric ambient pressure. Toxic gases are thus effectively prevented from entering the mask.
However, it is not necessary to generate an overpressure for each application. The air breathed in by the user of the mask directly is purified, for example, by a filter located at a breathing port in the “normal pressure” mode. The expiration valve is usually closed and opens only during the expiration phase of the mask user due to the expiration pressure developing in the mask. The pressure level within the mask corresponds to the level of the outside pressure in the “normal pressure” mode.
The pressure level within the mask is shifted in the “overpressure” mode by a certain positive amount compared to the level of the outside atmospheric pressure, so that the internal pressure in the mask is above the respective atmospheric pressure during both inspiration and expiration. The expiration valve opens during the expiration phase here as well due to the expiration pressure generated in the mask. In addition, the expiration valve must close against the now existing pressure gradient.
Thus, different pressures act on the expiration valve as a function of the mode of operation. The expiration valve must consequently be controlled corresponding to the respective mode of operation in case of a gas mask with the “normal pressure” and “overpressure” modes.
IT 1 227 248 discloses a spring, which counteracts the opening of the outlet valve and which acts on a pin projecting towards the outside of the mask. The pin is arranged such that it is pressed to increase the pressure of the spring when the internal pressure of the mask is increased, for example, by connecting a compressed air breathing apparatus.
A gas mask with two series-connected springs having different prestresses is known from EP 0 667 171 B1. The spring with the lower prestress is active when the mask is used in the filter operation, whereas the spring with the higher prestress is activated during use with a demand oxygen system, i.e., with overpressure in the mask.
DE 10 2004 052 173 B3 shows a gas mask of the type mentioned, in which a prestress of a valve spring can be varied by means of an adjusting means, wherein said adjusting means is designed as an angle lever pivotable about an axis of rotation.